1. Field of the Invention
The present invention relates generally to tuners employed to receive digital broadcasts, and particularly to tuners employed to receive ground wave digital broadcasts and digital cable television broadcasts.
2. Description of the Background Art
In North America, ground wave digital broadcasting employs a ground wave digital signal, which is a signal modulated in an 8 vestigial side band (8VSB) modulation system and having a frequency band of 54 MHz-806 MHz. The ground wave digital signal is transmitted from a TV station, and received by each household antenna and supplied to a television set, a VTR or similar AV equipment having mounted a tuner therein for receiving ground wave digital broadcasts.
FIG. 10 is a block diagram showing a configuration of a tuner employed to receive ground wave digital broadcasts, as conventional, for North America. In the figure, the tuner includes a casing 91, an input terminal 92, a ground wave tuner circuit 93, an 8VSB demodulation circuit 94, and an output terminal 95.
Input terminal 92 receives through a coaxial cable a ground wave digital received by an antenna and provides the signal to ground wave tuner circuit 93. From ground wave digital signal of a frequency band of 54 MHz-806 MHz, ground wave tuner circuit 93 selects only a signal of a desired channel and converts the signal to an intermediate frequency (IF) signal. 8VSB demodulation circuit 94 receives the IF signal from ground wave tuner circuit 93 and demodulates the signal in the 8VSB system to generate a transport stream (a multiplexed data signal as defined by MPEG2) and provide it to output terminal 95. Furthermore, 8VSB demodulation circuit 94 automatically controls ground wave tuner circuit 93 in gain so that ground wave tuner circuit 93 outputs a signal having a constant level. Output terminal 95 receives the transport stream, which is converted by an external signal processing circuit to video, audio, and data signals.
In North America, digital cable television (hereinafter referred to as “CATV”) broadcasting employs a digital CATV signal including a signal modulated by a quadrature amplitude modulation (QAM) system and having a frequency band of 54-864 MHz for forward application transport-channel (FAT-CH), and a signal modulated in a quadrature phase-shift keying (QPSK) system and having a frequency band of 70 MHz-130 MHz for forward data channel (FDC). Typically, a CATV station transmits a digital CATV signal which is in turn distributed through a coaxial cable to each household and supplied to a set top box rented from the CATV station for digital CATV. Furthermore, a CATV station having a bidirectional CATV system also employs an uplink signal modulated in the QAM or QPSK system and having a frequency band of 5 MHz-54 MHz for reverse data channel (RDC). This uplink signal is a signal for transmitting data to a CATV station from each household's set top box having a tuner mounted therein for receiving digital CATV broadcasts.
FIG. 11 is a block diagram showing a configuration of a tuner employed to receive digital CATV broadcasts, as conventional, for North America. In the figure, the tuner includes a casing 101, an input/output terminal 102, a coupling circuit 107, a distribution circuit 104, a FAT-CH tuner circuit 105, an FDC tuner circuit 106, a QAM demodulation circuit 107, a QPSK demodulation circuit 108, an RDC modulation circuit 109, a variable amplifier 110, output terminals 111 and 112, and an input terminal 113. This tuner accommodates bidirectional communication.
Input/output terminal 102 receives through a co-axial cable a digital CATV signal distributed from a CATV station. Coupling circuit 103 receives the digital CATV signal from input/output terminal 102 and transmits the signal to distribution circuit 104. Distribution circuit 104 receives the digital CATV signal from coupling circuit 103 and distributes the signal to FAT-CH and FDC tuner circuits 105 and 106. Distribution circuit 104 distributes the digital CATV signal's power equally.
FAT-CH tuner circuit 105 selects only a signal of a desired channel from digital CATV signals having the frequency band of 54 MHz-864 MHz (for forward application channel) and converts the selected signal to an IF signal. QAM demodulation circuit 107 receives the IF signal from FAT-CH tuner circuit 105 and demodulates the signal in the QAM system to generate a transport stream and provide it to output terminal 111. Furthermore, QAM demodulation circuit 107 automatically controls FAT-CH tuner circuit 105 in gain so that FAT-CH tuner circuit 105 outputs a signal having a constant level. Output terminal 111 receives the transport stream, which is in turn converted by an external signal processing circuit to video, audio and data signals.
FDC tuner circuit 106 selects only a signal arranged in the frequency band of 70 MHz-130 MHz for the forward data channel and converts the signal to an IF signal. QPSK demodulation circuit 8 receives the IF signal from FDC tuner circuit 106 and demodulates the signal in the QPSK system to generate a transport stream and provide it to output terminal 112. Furthermore, QAM demodulation circuit 108 automatically controls FDC tuner circuit 106 in gain so that FDC tuner circuit 106 outputs a signal having a constant level. Output terminal 112 receives the transport stream, which is in turn converted by an external signal processing circuit to an STB control signal applied to control a set top box (e.g., make a setting of whether a pay channel is receivable).
Input terminal 113 receives an externally transmitted data signal and passes the signal to RDC modulation circuit 109. RDC modulation circuit 109 receives the signal from input terminal 113 and modulates the signal in the QAM (or QPSK) system to provide a signal having the frequency band of 5 MHz-54 MHz (for the reverse data channel). Variable amplifier 110 receives the modulated signal from RDC modulation circuit 109 and amplifies it to attain an appropriate level. Coupling circuit 103 receives the amplified signal from variable amplifier 110 and transmits it to input/output terminal 102 as an uplink signal. Input/output terminal 102 receives the uplink signal and transmits it through the co-axial cable to a CATV station.
To promote ground wave digital broadcasting in North America, the Federal Communications Commission (FCC) has mandated TV receiver manufacturers as of July 2004 to mount in stages a tuner in broadcast reception devices to receive ground wave digital broadcasts. Furthermore, the FCC has also mandated mounting a tuner in digital broadcast reception devices to receive digital CATV broadcasts. Thus the tuner for ground wave digital broadcasts and that for digital CATV broadcasts must both be mounted in a broadcast reception device. As a result the device is increased in size, power consumption and price.
Japanese Patent Laying-Open No. 2001-285752 discloses a reception device accommodating multiple systems and easy to use for users. According thereto, it can change a screen smoothly for example in automatically switching to a demodulation circuit corresponding to a selected, modulated wave; changing digital and analog broadcasts seamlessly in channel; and/or switching an analog broadcast program to a digital broadcast program.
The conventional broadcast reception device having mounted both the tuner for ground wave digital broadcasts and that for CATV broadcasts thus has disadvantageously increased size, power consumption and price.
Furthermore, if the tuner for ground wave digital broadcasts and that for digital CATV broadcasts are implemented by a single shared tuner unit, substantially identical frequency bands are used, and isolation of an input terminal for a ground wave digital signal and that for a digital CATV signal (signal leakage degree) must sufficiently be considered. Furthermore, for the tuner for digital CATV broadcasts, a large number of channels including an adjacent channel is used. Accordingly, the tuner must be designed to prevent a distortion attributed to multichannel. By contrast, the tuner for ground wave digital broadcasts needs to be designed with noise figure (NF) significantly considered so as to accommodate a ground wave digital signal having a wide range in level. As such, it has been difficult to design a shared tuner unit receiving both ground wave digital and digital CATV broadcasts.